Bone mineral is composed primarily of a poorly crystalline nonstoichiometric apatite similar to hydroxyapatite, but also contains HPO.sub.4.sup.2- and CO.sub.3.sup.2- as well as a variety of cations. For example, it is known that HPO.sub.4.sup.2- ion concentrations are the highest in newly deposited bone and that their relative concentration decreases as the mineral matures. Thus the ability to distinguish between PO.sub.4.sup.3- and HPO.sub.4.sup.2- in vivo is of great biochemical interest because of its potential application to diagnosis and treatment of conditions such as osteoporosis, Paget's disease, and bone defects including fractures, grafts, and tumor resections. Other applications include the study of the resorption and remodeling of calcium phosphate-based bone cements and implants.
A Magnetic Resonance (MR) spectral editing technique proposed by Melchior is a spectrally selective solid state technique that has been used to resolve overlapping spectra in heterogenous, multicomponent polymer systems and organic compounds. It discriminates between species with overlapping spectra on the basis of their cross polarization times and allows selective resonances to be nulled. The technique has been variously called inversion recovery cross polarization (IRCP), cross polarization with polarization inversion (CPPI), differential cross polarization (DCP), and cross polarization--depolarization (CPD) in the literature. Wu et al. were able to use a DCP technique to suppress the dominant phosphate group and hence detect and identify a unique protonated phosphate present in bone mineral ex vivo.
Prior art DCP techniques have typically required the simultaneous irradiation of the sample at the resonance frequencies of the two nuclei with the magnitudes of both these fields much larger than the local dipole-dipole fields in the sample. These large fields can be difficult to generate over a large volume, and can present problems when applied to lossy samples such as biological tissues, as the RF power absorption scales with the square of the RF field amplitude and can produce tissue heating.